1. Field of the Invention
The present invention generally relates to data communications in which digital information is transferred in a burst formation, and more particularly to a multiplexed transfer system in which 1:1 or 1:n communications take place in the burst formation.
Conventionally, a two-wire cable is used as a transmission path or line connecting an exchange and a subscriber terminal. As transfer methods using such a two-wire cable, a two-wire time division transfer method, a two-wire hybrid transfer method and a two-wire frequency division transfer method are known. The two-wire time division transfer method is advantageous to the other two transfer methods in terms of the system configuration and the transmission rate, and is most attractive at present.
The two-wire time division transfer method is based on a 1:1 transmission. Recently, a two-wire time division transfer method based on a 1:n transmission has been proposed. In such transfers, it is necessary to temporarily store information to be transmitted in a buffer in order to assemble burst signals therefrom. In this case, a delay of transmission occurring in the above burst process is a problem to be solved.
FIG. 1 is a block diagram of a burst transfer system realizing a 1:1 transmission. Terminal equipments such as analog telephone sets and ISDN (Integrated Services Digital Network) terminals are accommodated in an exchange office via line termination equipments (which are also referred to as network termination equipments) 10 and transmission lines L. The transmission lines L are formed with coaxial cables or optical fiber cables. The exchange office includes an exchange 12, an in-office line termination equipment 14 and an operation/setting device 16. The in-office line termination equipment 14 accommodates the transmission lines L connected to the network termination equipments 10, and has the function of data multiplexing and demultiplexing. The operation/setting device 16 sets information that defines the operations of the in-office line termination equipment 14. The exchange 12 performs a data exchange. In FIG. 1, the network termination equipments 10 are represented as symbol ONU, and the in-office line termination equipment 14 is represented as symbol SLT for the sake of convenience.
FIG. 2 is a timing chart of the operation of the burst transfer system shown in FIG. 1. The operation shown in FIG. 2 is so-called ping-pong transfer. Each transmission line L is used in time division formation, and a data transfer from the exchange office to the terminal equipment and a data transfer from the terminal equipment to the exchange office are alternately very constant period T. In FIGS. 1 and 2, the direction from the exchange office to the terminal equipments is defined as a "down" direction, and a data burst transferred in the down direction is referred to as a down data burst. Also, the direction from the terminal equipments to the exchange office is defined as an "up" direction, and a data burst transferred in the up direction is referred to as an up data burst. FIG. 2 shows an operation that transmission information sent by the exchange office is sent to the corresponding transmission line L as down data bursts.
FIG. 3 shows a block diagram of a burst transfer system realizing a 1:n transmission. In FIG. 3, parts that are the same as those shown in FIG. 1 are given the same reference numbers.
As shown in FIG. 3, a branch device 18 is provided in the transmission line L. The branch device 18 branches one transmission line L to a plurality of transmission lines. By providing the branch device 18, the exchange office can view terminal equipments as if these terminal equipments are connected to one transmission line L (that is, network termination equipments). With the above structure, it becomes possible to efficiently use the transmission line L and accommodate a large number of terminal equipments.
FIG. 4 is a timing chart of the operation of the burst transfer system shown in FIG. 3. More particularly, FIG. 4 shows the operation of one transmission line to which n terminal equipments are connected via the branch device 18. The down data bursts from the exchange office are transferred in a broadcasting manner. Each of the down data bursts includes down data sequences 1 through n to be respectively sent to the n terminal equipments. Each of the down data sequences 1 through n are consecutive data. The n terminal equipments output n up data bursts to the corresponding transmission lines L at respective timings which are determined in a manner described later. Transferring of one down data burst and subsequent transferring of n data bursts are alternately performed every constant period T. The amount of information transferable during the constant period T by the 1:1 transmission is equal to that transferable by the 1:n transmission.
FIG. 5A shows a frame structure of down data bursts used in the 1:n transmission, and FIG. 5B shows a frame structure of up data bursts used in the 1:n transmission.
One frame of the down data burst shown in FIG. 5A is equal to a period Td (&lt;T), and includes a preamble pattern PR, a framing (frame synchronizing) pattern FR, overhead information OH and information Di (i=1, 2, . . . , n) arranged in this order. The preamble pattern PR, the framing pattern FR and the overhead information OH forms a redundant part (header part). The preamble pattern PR is pattern data for reproducing received data and a clock signal. The framing pattern FR is pattern data for detecting the constant period T. The overhead information is information indicating the state of use of the information area in a frame storing items D1 through Dn of information. The items D1 through Dn of information are items of information respectively sent to the terminal equipments from the exchange office.
One frame of the down data burst shown in FIG. 5B is equal to a period Tu (&lt;T), and includes a preamble pattern PR, a framing pattern FR, overhead information OH and information D arranged in this order. The framing pattern FR in the up data burst is a pattern indicating the beginning of the burst.
Each of the network termination equipments 10 shown in FIG. 3 reproduces the clock signal from the preamble pattern PR of the data burst transferred via the transmission line L, and detects the framing pattern. When a predetermined timing synchronized with the reproduced clock signal is obtained, each of the network termination equipments 10 starts to send a data burst as shown in FIG. 5B. The in-office line termination equipment 14 shown in FIG. 3 receives data bursts from the n terminal equipments, and reproduces clock signals from the preamble patterns PR of the received data bursts. Then, the in-office line termination equipment 14 receives the subsequent framing pattern FR and information.
FIG. 6 is a diagram of a circuit configuration which is related to formation of burst data, and is provided in each of the transmission lines L connected to the network termination equipments 10 and the in-office line termination equipment 14. Hereinafter, the circuit configuration shown in FIG. 6 is referred to as a burst circuit. The burst circuit shown in FIG. 6 is made up of two T-time data memory circuits 20 and 22, two switches 24 (SW1) and 26 (SW2), a burst formation circuit 28, a T-time generating circuit 30, a redundant part circuit 32 and an inverter 34.
The following description of the burst circuit is related to a case where it is connected to one transmission line L connected to the in-office line termination circuit 14. Transmission information from the exchange 12 shown in FIG. 3 is sent to either the data memory circuit 20 or the data memory circuit 22 by means of the switch 24. Assuming now that the switch 24 selects the data memory circuit 20, the above transmission information (which corresponds to the items D1 through Dn of information shown in FIG. 5A) addressed to the terminal equipments is written into the data memory 20. During this time, the switch 26 selects the data memory circuit 22, and the previously written transmission information is read therefrom. The control of the switches 24 and 26 is performed according to a timing signal output by the T-time generating circuit 30. This timing signal is applied directly to the switch 26, and is applied via the inverter 34 to the switch 24. The switches 24 and 26 are switched every T time so that data is written into one of the data memory circuits 20 and 22 and data is read from the other data memory circuit.
The burst formation circuit 28 receives the timing signal output by the T-time generating circuit 30, and then adds the redundant part to the transmission information received via the switch 26. As has been described previously, the redundant part includes the preamble pattern PR, the framing pattern FR and the overhead information OH. The data bus thus generated is output to the transmission line L in the frame format shown in FIG. 5.
The configuration shown in FIG. 6 provided in each of the network termination equipments 10 operates in the same manner as described above. In this case, the switch 24 receives transmission information from the corresponding terminal equipment.
However, in any of the above-mentioned 1:1 transfer and 1:n transfer, it is necessary to store information during the constant period T and send it to the transmission line L during the next constant period T. Hence, a delay of the period T occurs in the above process, and information is transferred with a delay of time.